It's magic.
I can't say for sure because I didn't have a hand in fixing it, but I'm guessing you either changed file formats there or the new signature is a more highly compressed JPEG.
The most obvious way to store an image is to give each pixel an address and a value for color. That is also the way that takes up the most space. That is what a bitmap (BMP) is. There are other ways smart people came up with that involve who knows what, including something called lossy compression. That cuts down on the file size. That's about as much as I can say without making myself look terribly ignorant. As it is, I'm sure somebody would be able to laugh at me.